The invention is directed to a dispersion compensating optical fiber and a transmission system including the same, and more particularly to a dispersion compensating optical fiber and transmission system in which the dispersion compensating fiber exhibits a negative dispersion and dispersion slope within the L-band (1570 nm to 1620 nm).
Higher data rates are becoming needed for the telecommunications industry. Thus, the search for high performance optical fibers designed for long distance, high bit rate telecommunications has intensified. However, these high data rates have penalties associated with them. In particular, dispersion is a significant problem for such systems, particularly those employing large effective area fibers. More specifically, positive dispersion builds as a function of the length of the high data rate transmission fiber. Dispersion Compensating (DC) fibers included in cable or in Dispersion Compensating Modules (DCM""s) have been designed that compensate for such dispersion. These fibers generally have negative slope and negative dispersion such that a short length of the DC fiber compensates for the positive dispersion and positive slope of the longer transmission portion. A good example of a DC fiber may be found in commonly assigned U.S. patent application Ser. No. 09/802,696 filed on Mar. 9, 2001. For L-band operation between 1570 nm and 1620 nm, the bend performance and dispersion properties (dispersion and/or dispersion slope) of the DC fiber are very important. This is particularly true in DC fibers that will be included in a wound spool of a DCM.
Thus, there is a need for a DC fiber which: (1) is single moded over the L-band wavelength range (1570 nm to 1620 nm) when included in a DCM; and (2) retains the usual high performance optical fiber characteristics such as high strength, low attenuation and acceptable resistance to bend induced loss, and (3) is particularly effective at compensating for the dispersion of Non-Zero Dispersion Shifted Fibers (NZDSF) in the L-band.
The following definitions are in accordance with common usage in the art.
The refractive index profile is the relationship between refractive index and optical fiber radius.
A segmented core is one that has at least a first and a second segment such as a central core and a moat, for example. Each core segment has a respective refractive index profile and maximum and minimum index.
The radii of the segments of the core are defined in terms of the beginning and end points of the segments of the refractive index profile or in terms of the midpoint of the segment in the case of a ring segment. FIG. 2 illustrates the definitions of radii used herein. The same definitions are used for FIGS. 3-5. The radius R1 of the center core segment 22, is the length that extends from the DC fiber""s centerline (CL) to the point at which the profile crosses the relative refractive index zero as measured relative to the cladding 30. The outer radius R2 of the moat segment 24 extends from the centerline to the radius point at which the outer edge of the moat crosses the refractive index zero, as measured relative to the cladding 30. The radius R3 is measured to where xcex943 % is half its maximum value of the ring segment 26. The half-height width of ring segment 26 is measured at the half xcex94 % value of ring segment 26. The radius R3 of segment 26 extends from the centerline (CL) to the midpoint 28 of a half-height line segment 27. The midpoint 28 is formed by bisecting the segment 26 between the two intersection points with the ring segment at the half height position of xcex943 %. The radius R4 is measured from the centerline (CL) to the point where the outermost portion of the ring segment 26 meets the zero refractive index point, as measured relative to the cladding 30.
The effective area is defined as:
Aeff=2xcfx80 (∫E2 r dr)2/(∫E4 r dr), where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light as measured at 1595 nm.
The effective diameter, Deff, is defined as:
Deff=(2/xcfx801/2)Aeff1/2 
The profile volume is defined as 2 xcfx80∫xcex94 % r dr. The profile volume of the central core segment 22 extends from the waveguide centerline, R=0, to the radius R1. The profile volume of the ring segment 26 extends from the radius R2 to the last point of the ring segment at radius R4. The units of the profile volume are % xcexcm2 because relative index is dimensionless. The profile volume units, % xcexcm2, will be referred to simply as units throughout this document.
The term, xcex94 %, represents a relative measure of refractive index defined by the equation,
xcex94 %=100(ni2xe2x88x92nc2)/2nc2 
where ni is the maximum refractive index in the respective region i (e.g., 22, 24, 26), unless otherwise specified, and nc is the refractive index of the cladding (e.g., 30) unless otherwise specified.
The term alpha profile, xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b) %, where b is radius, which follows the equation,
xcex94(b)%=[xcex94(bo)(1xe2x88x92[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1)]100 
where bo is the maximum point of the profile and b1 is the point at which xcex94(b)% is zero and b is in the range bixe2x89xa6b greater than bf, where xcex94 % is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number. The initial and final points of the xcex1-profile are selected and entered into the computer model. As used herein, if an xcex1-profile is preceded by a step index profile, the beginning point of the xcex1-profile is the intersection of the xcex1-profile and the step profile. In the model, in order to bring about a smooth joining of the xcex1-profile with the profile of the adjacent profile segment, the equation is rewritten as;
xcex94(b)%=[xcex94(ba)+[xcex94(bo)xe2x88x92xcex94(ba)]{(1xe2x88x92[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1}]100, where ba is the first point of the adjacent segment. 
The pin array bend test is used to compare relative resistance of optical fibers to bending. To perform this test, attenuation loss is measured when the optical fiber is arranged such that no induced bending loss occurs. This optical fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The optical fiber is caused to pass on opposite sides of adjacent pins. During testing, the optical fiber is placed under a tension sufficient to make the waveguide conform to a portion of the periphery of the pins.
The DC fiber in accordance with the invention disclosed and described herein is particularly well suited to compensating for dispersion and dispersion slope of certain NZDSF in the L-band.
According to an embodiment of the invention, a DC fiber is provided which has a segmented core of at least three segments, each segment characterized by having a refractive index profile, a relative index xcex94 %, and radius dimensions. The DC fiber""s overall refractive index profile structure is selected to provide a particular set of properties (attributes) that make it suited for transmission systems designed to operate in the L-band wavelength window having a midpoint at about 1595 nm, and a wavelength band between about 1570 nm and 1620 nm. The DC fiber in accordance with the invention is particularly suitable for compensating for build up of dispersion and/or dispersion slope in NZDSF""s. Thus, the DC fiber may be coupled to a NZDSF to form a transmission system and is designed to compensate for the dispersion and/or slope (and most preferably both) of the NZDSF, preferably in the L-band. The transmission system including the DC fiber may also preferably include optical amplifiers, Wavelength Division Multiplexing operation, and other conventional system components. Preferably, the DC fiber is wound onto a spool and included in a module.
In accordance with an embodiment of the invention, the total dispersion (defined herein as the measurable dispersionxe2x88x92total dispersion equals total dispersion plus waveguide dispersion plus profile dispersion) of a transmission system employing 100 km of a NZDSF transmission fiber and a suitable length of the present invention DC fiber results in a system which has less than +/xe2x88x9225 ps/nm residual dispersion over the entire L-band (between 1570 nm and 1620 nm). Fiber profiles have been designed in accordance with the invention that exhibit excellent attenuation of less than 0.8 dB/km at 1595 nm. Moreover, the bend loss, as measured by a pin array test, is preferably less than 25 dB, more preferably less than 10 dB, and most preferably less than 3 dB. Thus, the DC fiber in accordance with the invention exhibits excellent bend loss and may be, therefore, advantageously wound onto and used in small diameter DCM""s to be utilized in transmission systems for compensating dispersion and dispersion slope of long lengths of NZDSF.
In accordance with a preferred embodiment of the DC fiber, each of the segments of the core is characterized by a refractive index profile, and at least one of the segments preferably has an xcex1-profile. Most preferably, the core profile includes a positive xcex941 % central core segment, a negative xcex942 % moat region, and a positive xcex943 % ring segment. Preferably, the ring segment has a non-step index profile and is offset from the moat segment.
According to the present invention, the DC fiber has a segmented core having at least three segments and the refractive index profile of the segmented core is selected to provide a negative total dispersion and a negative dispersion slope at 1595 nm, and more preferably over the entire L-band from 1570 nm to 1620 nm. The present invention DC fiber has a total dispersion at 1595 nm between about xe2x88x9270 ps/nm-km and xe2x88x92225 ps/nm-km at 1595 nm; and a dispersion slope more negative than xe2x88x920.7 ps/nm2-km at 1595 nm. More preferably, the dispersion at 1595 nm is between about xe2x88x9295 and xe2x88x92225 ps/nm-km; and the dispersion slope more negative than xe2x88x920.9 ps/nm2-km at 1595 nm. More preferably yet, the dispersion at 1595 nm is between about xe2x88x92110 and xe2x88x92150 ps/nm-km and ranges between xe2x88x9280 and xe2x88x92190 ps/nm-km over the L-band wavelength range of 1570 nm to 1620 nm.
Most preferably, the dispersion slope is more negative than xe2x88x920.70 ps/nm2-km at 1595 nm and is preferably between xe2x88x920.9 ps/nm2-km and xe2x88x921.5 ps/nm2-km at 1595 nm.
Preferably also, the DC fiber has a dispersion slope that is more negative than xe2x88x920.5 ps/nm2-km over the entire L-band from 1570 nm to 1620 nm; more preferably more negative than xe2x88x920.7, and most preferably less than xe2x88x921.2 ps/nm2-km. Preferably, the dispersion slope ranges between xe2x88x920.5 and xe2x88x922.5 ps/nm2-km over the entire L-band; more preferably between xe2x88x921.0 and xe2x88x921.8 ps/nm2-km.
The DC optical fiber preferably has a kappa value defined as the total dispersion at 1595 nm divided by the dispersion slope at 1595 nm of between 90 nm and 110 nm; more preferably between 90 nm and 105 nm; and most preferably between 95 nm and 100 nm. Most preferably, kappa is between about between 80 nm and 155 nm over the L-band range of 1570 nm to 1620 nm; more preferably between 85 nm and 110 nm.
The DC fiber preferably includes a central core segment having an a-profile in the range of between about 1.8 to 5.0; more preferably about 2.0 to 2.2.
The DC fiber in accordance with embodiments of the invention preferably has a central core segment having a positive xcex941 % greater than 1.5%, a moat segment adjoining the central core segment and having a negative xcex942 % more negative than xe2x88x920.3%, and a ring segment adjoining the moat segment having a positive xcex943 % greater than 0.6%.
More preferably, the DC fiber in accordance with embodiments of the invention preferably has a central core segment having a positive xcex941 % greater than 1.7%, a moat segment adjoining the central core segment and having a negative xcex942 % more negative than xe2x88x920.5%, and a ring segment adjoining the moat segment having a positive xcex943 % greater than 0.8%.
The effective area of the DC fiber at 1595 nm in accordance with the invention is greater than 15 xcexcm2, and more preferably greater than 17 xcexcm2.
In accordance with another embodiment of the invention, an optical transmission system is provided having a dispersion compensating optical fiber, wherein the dispersion compensating fiber comprises a segmented core having at least three segments, the refractive index profile being selected to provide a total dispersion at 1595 nm between about xe2x88x9270 ps/nm-km and xe2x88x92225 ps/nm-km; and a dispersion slope more negative than xe2x88x920.7 ps/nm2-km at 1595 nm.
Further features and advantages of the invention will be set forth in the detailed description which follows, and will be readily apparent to those of ordinary skill in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate several embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.